The present invention relates to methods of forming oxide films by reactive sputtering.
Thin oxide films are used as coatings on optical elements such as lenses, mirrors and window panes. These thin films can be used in known fashion to provide particular mechanical, chemical and optical properties. In certain applications, it is desirable to provide a film of a substantially pure oxide such as titanium dioxide, silicon dioxide, tin dioxide or aluminum oxide. Such substantially pure oxide films offer good resistance to mechanical abrasion, resist degradation by moisture in the environment and provide films with indices of refraction ranging from 1.45 (SiO.sub.2) to 2.40 (TiO.sub.2). Accordingly, a substantially pure oxide film may be utilized as an exposed surface of an optical element. For example, a window pane may be provided with a multilayer coating incorporating layers of titanium dioxide and layers of silicon dioxide in an alternating arrangement to provide transmission and reflection of light at particular wavelengths.
Pure oxide films may be deposited by a process known as reactive sputtering. In sputtering, the item to be coated or "substrate" is placed adjacent a source of material to be deposited, commonly referred to as a "target". The target is exposed to a plasma or ionized gas and the target is placed at a negative electrical potential with respect to the plasma. Under these conditions, ions from the plasma strike the target with considerable energy and dislodge material from the target. The dislodged material deposits on the substrate to form the coating. Ordinarily, the entire sputtering process is conducted within a chamber maintained at a low, subatmospheric pressure. In reactive sputtering, the plasma contains a reactive constituent which combines with the material of the target so that the deposited coating includes both the material from the target and the reactive constituent from the plasma. Thus, oxide coatings are commonly formed by reactive sputtering utilizing a target containing a metalloid such as silicon or a metal such as titanium, tin or aluminum together with a plasma containing oxygen. The oxygen in the plasma combines with the metal or metalloid dislodged from the target, and the coating deposited on the substrate is an oxide of the metal or metalloid.
Reactive sputtering processes according to this general scheme can provide high quality, substantially pure oxide films. However, the speed of the sputtering processes employed to deposit oxide films heretofore has been limited. The cost of applying a particular film by the sputtering process is directly related to the time required for the process. Accordingly, considerable effort has been expended heretofore in the refinement of the reactive sputtering process to enhance its speed. Although considerable progress has been made heretofore, there has been, nonetheless, a long felt need for a reactive sputtering process capable of depositing a high quality, substantially pure oxide film at higher speed.
It has been known heretofore that films including metal, oxygen and fluorine can be deposited by a reactive sputtering process utilizing an atmosphere containing both oxygen and fluorine. Such processes are disclosed in U.S. Pat. No. 3,477,936 and in Harding, "High Rate DC Reactively Sputtered Metal-Oxy-Fluorine Dielectric Materials", Thin Solid Films, Vol. 138 (1986) 279-287. Harding noted that such a deposition process proceeds at a relatively rapid rate. However, the fluorine-containing films made by these processes have optical, mechanical and chemical properties differing from the properties of the pure oxides. These references do not teach that their processes can provide substantially pure oxide films.